Custom Layout

I need to setup a template so that has left, center and right column boxes but below the left and center columns have a full width sidebar below. The full width sidebar cannot go across the right column. (See image below). Are there any suggestions on how I best go about doing this and make user friendly?

Given the description above, I'm not sure which of the two scenarios below would work for you. With a Universal or Full-Width SB, you could add a text widget, then include a grid limiting the content of the grid to fit a specific area.

I showed this layout to the person who will be managing the site and they said that with their previous site they could open the page and see the content as it would look when a visitor comes to the site. They would prefer to manage that way instead of the way I designed earlier.

Can I setup each page so that when an editor logs in to make changes they can see the content (in our case 3 columns and 3 rows) and make changes? Maybe using the visual tab?

Would I be able to achieve that using grids? If I do use grids, how would I get the "twitter all in one" widget to run in the second column and my other widget to run in rows 2 & 3?

Widgets are controlled by WordPress via Dashboard > Appearance > Widgets. If you added Primary Sidebar to column 1, then you can add a widget (by drag & drop) into Primary Sidebar. Similarly, you can do the same thing with Secondary sidebar in column 2, etc.

To add Twitter Profiles, go to Dashboard > PageLines > Drag & Drop, find the section you want to add it to, then drag it from Available Sections to Active Sections. On my site, I use Sidebar Wrap, which provides a bit more room for this section.

Well, not exactly. You can use shortcodes to display plugins, provided the plugin has a shortcode to use for such purposes. When the plugin uses shortcodes for display, you can add them almost anywhere. If you want such shortcodes in the sidebar, I use a text-widget, and drop the shortcode into that and save.

If a plugin is designed to only use a widget, then the widget will appear in Dashboard > Appearance > Widgets and is limited to being placed in sidebars. Luckily, PageLines smartly created sidebars that can be placed inside the content area, allowing you to pretty much put things anywhere.